This invention relates to wireless security systems that use send-only radio frequency (RF) sensors to indicate a fault condition in the monitored area, and in particular to wireless security systems that cannot be armed when the status signifies a fault condition. The fault condition may be detected motion, detected glass breakage, etc.
Many low cost wireless security systems use send-only RF sensor transmitters to communicate status to a receiver/control unit. The receiver/control unit monitors the status by receiving and decoding messages. When the security system is armed and a fault has been indicated in a message, the receiver/control unit initiates an alarm. For example, a motion detector transmits a fault message when motion has been detected and a restore message when the motion has ceased. Each message is generally transmitted a number of times to provide a high probability of receipt by the receiver/control unit.
Most sensor transmitters are rarely activated concurrently in typical wireless security systems, therefore the loss of a message is unusual. This may not be true for large security systems, such as those installed in a school building. In a site such as this, motion detectors are typically used to provide maximum coverage in areas that are difficult and more expensive to protect using door or window-type sensors. During the day, when the security system is not armed, many motion detectors are concurrently activated by people moving within the protected areas (i.e. changing of classes between class periods). This results in many concurrent message transmissions to the receiver/control unit, causing interference or clashing and possibly memory overflow. A loss of a restore message causes the controller to maintain a sensor faulted status. When the receiver/control unit detects a faulted status, it will not allow a user to arm the security system. In order to subsequently arm the security system, someone must re-activate the motion detector that has the faulted status associated with it. This causes a restore message to be transmitted without being interfered with by other transmitted messages, thereby causing the faulted status to be cleared. It may be possible that a number of motion detectors have failed to successfully deliver their restore messages. Each detector that has a fault status associated with it will need to be activated in order to arm the system.
It is therefore an object of the present invention to provide a method for arming a send-only RF security system regardless of lost restore messages due to clashing messages and/or RF receiver buffer storage limitations.